Smart Power Measuring Sensor

The Smart Power Measuring Sensor (SPMS) uses a dedicated microprocessor in the package to provide analog to digital conversion, calibration, and preparation of serial data for delivery to a host computer over a two wire modulated carrier system. Power is obtained from the voltage being measured and a current sensor is built into each unit. Data is transmitted over a standard interface such as RS232 or RS485. Transformer coupling is used to provide isolation required by regulations.

Smart Sensors are potted environmentally rugged units. Each contains an inductive sensor of patented SRT design. The output of the sensor is scaled, compensated for temperature and phase, and synchronized with the line to enable measurements through the 64th harmonic. Multiple sensors can be connected for three phase power monitoring.

The SPMS units are programmable by the user for special tasks. A host computer downloads software to the sensor and then makes periodic requests of each sensor and uses the results as required. Calibration constants and a unique identification are permanently stored in ROM within each sensor. Code and data are stored within each sensor in nonvolatile 32K RAM which retains data for recovery by a host following a power failure. The SPMS measures both voltage and current for a stand alone system where single and three phase monitoring is needed. The SPMS is excellent for monitoring instantaneous and rms AC voltage, Power Factoring (real power vs. reactive power), energy consumption, current and voltage wave form samples up to 20 kHz, and instantaneous peak voltage to 20 times full scale. Software for the SPMS is downloaded by the host and may be rewritten in less than two seconds. Boxcar integration with samples synchronized with phase and voltage averaged will allow precision which increases with time. Five seconds corresponds to 16 bits at 60Hz.

Applications include: Features include:
  • Compare actual usage to historical data
  • Record characteristics of transients
  • Measure harmonic currents to fractions of an ampere
  • Check out user-to-utility interface
  • Check out reasons for opening of circuit breakers
  • Identify out-of-spec usage by motors and other equipment
  • Measure/monitor single or multiple variables in a Wye or Delta System (to 600A input).
  • Digital output is in international standard units; no calibration is required (stored inside each sensor).
  • Meets Federal (DOE) requirements for energy management.
  • Software downloaded to sensor can be modified by user.
  • Frequency: 25Hz to 3kHz; 32nd harmonic of 60Hz.

We've made available a downloadable .PDF file which illustrates how the SPMS is configured for three phase wye and delta systems. To view .PDF files you need Adobe Acrobat Reader. Adobe Reader is freeware which can be downloaded off their internet site at

If you wish to inquire about our Smart Sensor, you might want to look at our short SPMS questionnaire first. It's a list of the type questions we would need answered for an SPMS installation.

Engineers at SRT are available to discuss any questions you might have on any of our products. Often we are asked to customize or prepare special calibrations on our sensors to suit customer needs. Please contact us at (800) 447-6805 and ask for Dayle Smith or Doug McNutt, or FAX us at (719) 634-2601, or send E-mail to

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